I struggled with random Medallion gauge and MDC issues for the last 4 years (Since I bought the boat). The tach would drop to 0 RPM while running and stop counting hours, the temp gauge would peg to 240+ degrees then go back to normal after a “reboot” (Super scary the first 2 times) then stopped working completely and the fuel gauge never worked right. All connections were triple checked and cleaned with no improvement. I never could gather enough concrete evidence to make me comfortable that it was definitively either the MDC or the gauges themselves even working through the Medallion troubleshooting guide. The MDC is about $290+ shipping and individual gauges that matched mine never really showed up on eBay and if they had they would have been used and still not necessarily “known good”. There were full sets available new from later year models for a few hundred bucks but there’s so much mystery around this electronic bus system that it was hard to know for sure if they’d work with the older MDC. Pretty much anyway I went on this would have been a bit of a gamble so I decided to go with the cheapest gamble of them all: Eliminate the middle man (The MDC) and go with an aftermarket set of analog gauges. I had heard anecdotal reports of this being a pretty easy project but hadn’t been able to find a write-up or anything.

Changing gauges in general is a bit of a painful process which will take around 2-3 hrs to complete even if you were swapping in OE gauges. Swapping in aftermarket analog gauges and eliminating the MDC adds about 45 mins to an hour to this job to re-work the wiring harness. So, it’s about a 3, maybe 4 hour job depending on your familiarity with dealing with such things.

So, here’s what worked for me. Some of this might be specific to the 1st gen Maristar 230 VRS/X-30 but it’s probably close on just about any of the boats of 2000-2005ish. The pre 2000 boats didn't have an MDC or Medallion gauges so, although I haven’t seen behind their dashes, we’re electrically duplicating the setup that the older boats would have.

Parts/Supplies:
-Set of analog gauges for standard American Marine senders. I chose Faria KTF001 which is a 6 gauge set of white faced gauges. I had no need for the speedometer as I have Perfect Pass but it’s cheaper to buy the set with the speedometer than to buy the 5 gauges I needed individually. I paid $172.12 shipped. Teleflex or any other brand should work just as well.
-5-6 waterproof heatshrink 1/4” female “quick connects”
-14-16 waterproof heatshink #10 ring terminals
-2 heatshrink butt splices
-Small zip ties
-1/4” heatshrink tube

Tools:
-Small socket set or wrench set for removing nuts on gauges. Sizes varied but 3/8” was the largest.
-Wire Cutters
-Crimpers
-Lighter to shrink the tubing/crimp connectors
-Multimeter (Optional but you probably want to check to make sure your boat’s wire colors and what not are the same as mine)

1) Disconnect the battery’s negative cable
2) Remove the gauge panel:
Lie under the dash looking up and get comfortable. You're about to contort your arms and work by feel for about 5-10 minutes while being very uncomfortable. Reach up into the cavity behind the gauges and feel around. You should be able to locate 8 wing nuts that are threaded onto studs on the back of the gauge panel. Look at the stud locations in step 4 to get an idea. Each will have a very tiny lock washer and a larger fender washer behind the wing nut. Try to grab them when you remove the nut but if things fall down in there you can retrieve them after the gauge panel is removed. After you have all 8 wing nuts removed you should be able to pull the gauge panel off with all of the gauges on it.

Alternatively you can first remove the whole vinyl coated fiberglass dash by removing those wing nuts but I found that that's much more of a pain vs. just reaching for the wing nuts if all you need to access is the gauges themselves. I’ve done this at least 4-5 times now.

3) Disconnect all gauges and check engine light and remove the gauge panel. These are all removable connectors so no need to cut anything yet.
4) Swap your aftermarket gauges on the panel. This is pretty self-explanatory. The new tach had a little selector where I had to select the right position for an 8 cylinder engine.
5) Cut off all of the 5 pin gauge connectors where the old gauges connected. This is the point of no return.
6) Remove the yellow wire from the harness or just tape/heatshrink them and leave them in. This is the signal/bus wire and is no longer needed. I chose to remove it and “rebuild” the harness (More on that later)
7) Disconnect and remove the MDC. There are some short adapter pigtails for the 18 pin connector and the 5 pin connector. I just left those adapter pigtails on the MDC. If not using a speedometer plug or remove the pitot tube (unless you like wet feet/carpet)
8) Remove the plastic wire loom from the MDC harness
9) Pull the MDC and gauge harness up through the hole in the dash (Where the gauge panel normally resides)
10) Cut off the connector with the gray, pink, tan, light blue, etc wires (The upper one in the picture above). I'll refer to this as the "MDC Harness" from here on.
11) Extend the tan wire over to the temp gauge position
12) Extend the gray wire over to the tach position
13) Extend the light blue wire over to the oil pressure gauge position
14) Extend the pink wire over to the fuel gauge position
Note: On steps 10-13 there was way more wire in the factory harness than I would need. I did not need to add any additional wire for any of this project. The orange wire is not used. Cut it off/coil it up and tape or heat shrink it off so it doesn't ground out.
15) Cut off the gauge harness connector with the black/yellow stripe, red/white stripe, etc. wires on it (The lower one in the pic above). I'll refer to this as the "Gauge Harness" from here on.
16) Butt Splice the black wire (Ground) from the MDC harness to the black/yellow stripe wire from the Gauge Harness.
17) Butt splice the purple wire (Ignition switched +12v) from the MDC harness to the red/white stripe wire from the Gauge Harness.
Note: Steps 16 & 17 are necessary because with the MDC in place ground and ignition switched power were passed through the MDC. With that removed you need to re-complete those circuits. I had a few feet of extra wire between them that was removed during that process as the wire didn’t need to go down to the MDC on the kick plate and then back up to the gauges.
18) Strip and crimp on a 1/4” female on each dark blue wire in the gauge harness. This is tied into the NAV light switch and is for the gauge face backlights. There were warnings on the gauges to not allow this wire to touch the sender signal wire so I opted to put heat shrink over the terminal itself to insulate it from adjacent terminals.
19) Crimp #10 ring terminals on each Black/Yellow Stripe (ground), Red/White Stripe (ignition switched power), and the appropriate signal wire for that gauge position (i.e. gray for tach, pink for fuel, tan for temp, light blue for oil pressure). The volt meter doesn’t have a “signal” wire just +, - and + for backlight
20) Clean up your harness and prepare to reinstall the gauges. I had to re-terminate power and tach signal for my Perfect Pass as those were previously spliced in further down the line. I put heat shrink over the speedo wires since I wasn't installing it and the PP doesn't use those.
21) Connect all gauges per the instructions
22) Reconnect battery and test basic function
23) Close everything up by reversing the process in step 2

I tested by running it at home and all of my gauges work now except the fuel level. I apparently have a bad fuel level sender on top of everything else as when I test it with a multimeter it shows 270 ohms (i.e. very empty) regardless of fuel level (It’s supposed to be 33.5 ohms full and 240 ohms empty). I’ll be replacing that sender and I’ll hopefully be done with gauge issues for a few years. If nothing else at least this setup is 50% easier to troubleshoot without the MDC in the mix. It’s either the sender or the gauge now. With the MDC/Medallion setup with any “gauge” malfunction it could be the MDC, the gauge, the sender or even another gauge causing “noise on the bus”.

Technically this should work fine for the 2006+ newer boats too but with the billet and molded plastic dash panels you may have cosmetic issues making these look right. I’ll leave that up to some of you guys with the fancier boats to figure out. I’m also unsure if ballast gauges would come into play on those boats as well.

Although completely unrelated to the MDC my depth sounder was generally unreliable. I replaced it with a matching one in the process.

The look and feel of the gauges is of similar quality to the OE Medallion gauges. I think the overall look has been improved in the process for my particular boat. Now everything matches. I'll report back when I have the new sender installed and I've done an actual water test.

In the end it cost me about $185 with the terminals and what not (Not counting depth sounder or the broken fuel sender) to replace my 5 gauges. I likely can sell the unused speedometer and pitot kit to recoup some of that cost. That's over $100 cheaper than just the MDC or an old stock set of medallion gauges and certainly cheaper than gauges + MDC by a large margin.

Before:

After:

After with engine running and backlights on:

This little guy was instrumental in making this project take as long as possible:

I left it on eBay for a few weeks and finally sold the speedometer for $30. I thought I'd get more but that brings the total cost for the new gauges to $142.12 plus a few bucks for crimp connectors. That doesn't count the depth sounder but that system is unrelated to the MDC and wouldn't need replacement unless it happened to be bad too.

Looks as if I am going to have to follow in your steps sometime soon. All the gauges packed in for about 30 mins then came back to life. Interestingly when cycling the ignition the gauges were twitching so my guess a duff gauge is causing all the others not to read. Intermittent problems are the worst to deal with, so I think my time is best spent updating to analogue. Winter project I think.